1. Field of the Invention
This invention relates generally to insulation testers, and more specifically to insulation testers for electrical equipment.
2. General Background
Motors and generators are used in many industries and institutions. Offshore drilling rigs typically have one or more generators to meet their power requirements, as well as motors to perform various tasks on the rigs. Hospitals typically have backup generators for use in the event of a power outage.
A common cause of malfunctioning of electrical and electronic equipment is inadequate electrical insulation of the wiring, which can be due, for example, to manufacturing defects of the wiring, shorts introduced during manufacture of the equipment, or breakdown of the insulation.
There exist reliable methods of and devices for testing the soundness of electrical insulation of insulated wires prior to use in wiring systems and electrical and electronic equipment, both during and immediately after manufacture of the insulated wires. Examples of such devices and methods are disclosed in U.S. Pat. Nos. 3,413,541; 3,546,581; 3,789,294; 3,789,295; 3,823,370; 4,313,085; and 4,160,947.
There are also numerous means for testing electrical equipment to detect any problems with electrical insulation that may have been introduced during manufacture of the equipment. U.S. Pat. No. 4,152,640 discloses a method of testing the integrity of insulation of windings in newly manufactured electrical equipment by applying a positive DC voltage to both ends of one winding, a negative DC voltage of the same magnitude to both ends of another winding, grounding all other windings, and measuring the current flowing in series in the two windings being tested. The process is repeated until each winding has been tested with a positive and a negative voltage. A similar method is disclosed in which AC voltages, which are of the same magnitude but out of phase, are applied to the two windings being tested.
Even if the characteristics of the electrical insulation of wiring passes muster during testing prior to incorporation into equipment, and during testing before the equipment is put into use, there is no guarantee that the insulation will hold up indefinitely. Typical safeguards against damage to equipment due to insulation breakdown which occurs when the equipment is operative include fuses and circuit breakers which limit the amount of current passing through the equipment. Some non-grounded AC equipment is provided with "ground detectors" which constantly and automatically check the quality of the insulation of wiring by monitoring leakage current between the wiring and ground while the equipment is running, but which are inoperative when the equipment is not in use. Other devices to check insulation when equipment is running include a device disclosed in U.S. Pat. No. 4,214,311 which constantly monitors insulation resistance values in non-grounded DC equipment while the equipment is in operation, and one disclosed in U.S. Pat. No. 4,394,615 which monitors insulation between wiring and the housing of equipment when the equipment is on.
All too often, the potential for failure of electrical equipment caused by inadequate insulation occurs when the equipment is idle, due to insulation breakdown. Some common causes of insulation breakdown, in addition to thermal aging, include the presence of moisture, oil, grease, chemical fumes, and airborne contaminants. If the equipment is not tested until it is running, the testing may be useless because, if the insulation breakdown is serious enough, the equipment may fail and even explode on starting.
It is common practice today to occasionally check the insulation of motors and generators when the equipment is not running. A common way to do this is to open the motor or generator, find an exposed wire and check the resistance of the motor or generator by applying a DC voltage to the wire, and measuring the current which flows to ground (the chassis of the motor or generator). In the case of a permanently grounded system, the ground wire must be disconnected before the DC voltage is applied to a winding or a wire. Whether checking a grounded or ungrounded system, this method of insulation monitoring is time consuming and, therefore, is not performed as often as it should be. The inevitable result is that insulation breakdown goes undetected, and is often not discovered until a motor or generator fails when it is started, at great expense to its owner. Various solutions have been proposed to do away with the need for manually checking the insulation characteristics of motors when not in use. For example, U.S. Pat. No. 3,611,036 discloses a ground detecting circuit for a motor wherein two terminals of the motor are connected to two input power lines through diodes in series with neon lamps and current-limiting resistors. A ground (electrical contact between the wiring or windings and the chassis) in the motor produces a current flow through one or both of the lamps. A photosensitive resistor, in response to light from the lamps, activates a relay control circuit to open the input line to the motor so that the motor cannot be turned on.
U.S. Pat. No. 3,656,136 discloses a sensing device which is connected to one of the winding terminals of the motor when the motor is not operative. A voltage is applied to the winding terminal of the motor, and the current flowing through the terminal is monitored. When it rises above a predetermined level, a control signal is generated, activating a relay which causes contacts between the winding terminals and the input power lines to open, thereby preventing the motor from being turned on.
U.S. Pat. No. 4,319,297 discloses a protective system for motors in which a sensing means is coupled to a motor winding to detect and signal when the winding resistance drops below a predetermined level. Means are provided to decouple the motor winding from the sensing means when the motor is turned on.
While the solutions proposed in these three last-mentioned patents may be useful for detecting when the resistance of the windings of motors reaches an unacceptable level, they do not test the insulation, and only provide an indication of whether the resistance is above or below a certain, predetermined value. Also, they cannot be used with grounded motors or generators.